Hydrogen produced from sunlight: New record efficiency in direct solar water splitting

In a sustainable energy system, hydrogen will play a central role as a storage medium and as fuel for maintaining mobility. An international team of researchers has succeeded in increasing the efficiency of direct solar water splitting, the first step of natural photosynthesis, to over 19% by combining a tandem solar cell from III-V semiconductors with rhodium nanoparticles and crystalline titanium dioxide [1]. Teams from the TU Ilmenau, the Fraunhofer Institute for Solar Energy Systems, the California Institute of Technology and the University of Cambridge participated in this research.

Solar energy is available in great abundance worldwide but not at any location at any time. A solution approach for the issue of storage and also mobility is the storage of sunlight in the form of ″chemical energy“, so to say as solar-generated fuel. As every plant does, photosynthesis can also be performed with the help of artificial systems based on semiconductors; in practical terms, this means producing hydrogen with the help of sunlight through artificial photosynthesis.

In this process the electric power generated by the sun splits water into oxygen and hydrogen in a photo-electrochemical cell regarded as an artificial leaf. Hydrogen provides the highest energy density among the common fuels; it is very versatile, non-toxic and may entirely replace fossil fuels. Hydrogen can be easily stored and variedly used e.g. in a fuel cell for producing energy and heat or as a basis for other fuels. When combining solar cells with catalysts and other functional layers to form a ″monolithic photo-electrode“ building block, splitting water is especially simple: the photocathode is in an aqueous medium, and when exposed to light, hydrogen appears on the front and oxygen on the back.

Structure of a photocathode: Light passes through the transparent protective layer (TiO2) with catalytically active rhodium particles in the tandem cell, and water is split into its elements. Hydrogen forms on the front side and oxygen on the back.

Fig. ACS Energy Letters [1]

The combustion of hydrogen does not release climate-damaging carbon dioxide but only water. The production of ″solar hydrogen“has failed at the industry level until now due to costs. The efficiency of the artificial photosynthesis, i.e. the energy content of hydrogen in terms of the energy content of light, is still too low for economically producing hydrogen with the help of sunlight. For many years important centers of science have been conducting research worldwide to increase the efficiency and stability of artificial photosynthesis and to reduce costs. The TU Ilmenau has now repeatedly made a decisive contribution to this together with renowned research teams. In 2015 the TU Ilmenau and its partners significantly increased the existing record, which was valid for 17 years, from 12.4 to 14 percent as published in Nature Communications[2]. Now that the National Renewable Energy Lab has again been able to outperform this figure, the research team has recently further boosted efficiency impressively [1].

Efficiency 19.3 percent

Scientists achieved a level of efficiency of 19.3 percent (in diluted aqueous perchlorid acid), and respectable 18.5 percent in (neutral) water when exposed to simulated solar irradiation. This is already very close to the theoretical maximum efficiency of about 23 percent, which can be achieved with this combination of layers due to their electronic properties. The researchers have combined a highly efficient tandem cell made of III-V-semiconductors and developed at the Fraunhofer ISE with other functional layers to receive the jointly developed monolithic photocathode. The performance demonstrates that tailor-made tandem cells for direct solar water splitting have the potential to achieve an efficiency level higher than 20 percent. The research team succeeded in significantly reducing losses by light reflection and absorption on the surface. This is one of the reasons for the innovation. As early as 2015, the research team was able to achieve an efficiency level of over 14 percent in an earlier developed cell, a world record at the time. In the most recent approach, the anti-corrosion layer was replaced by a crystalline titanium dioxide layer, which not only provides excellent anti-reflection properties, but also adheres to the catalyst particles. In addition, a new electrochemical process was used to deposit the rhodium nanoparticles that act as catalysts for water splitting. They measure only around 10 nanometers in diameter and are therefore optically almost transparent, i.e. they are very useful for their task.

Improvement of stability: Translucent corrosion protection

The crystalline titanium dioxide layer does not only protect the actual tandem cell from corrosion but also improves the charge transport due to its favourable electronic properties. The stability of the water-splitting cell was increased to almost 100 hours - a major improvement over previous systems, which had already corroded after 40 hours. However, much remains to be done. In the component structures existing so far, losses in performance characteristics and stability remain precariously and incompletely understood. This is now to be addressed in a consortium of five research institutions supported by the German Research Foundation (DFG) and initiated by the Technische Universität Ilmenau. After the clarification, new criteria for the design of functional interfaces, contacts and passivation layers can be determined as well as innovative concepts for the stable linkage of available catalysts under changing operating conditions.

Aims within sight: Tandem cells with silicon

Last year, Dr. Matthias May, Prof. H.-J. Lewerenz (†) and Prof. Thomas Hannappel, together with the invention registration "Membrane tube arrangement for direct solar water splitting", received a gold medal for the TU Ilmenau at the international inventors' fair iENA. However, this is still basic research with small, high-priced systems in the laboratory, but researchers are optimistic to achieve sufficient stability and an even higher efficiency level. A new approach would be an even better combination of the band gap energies in the absorber materials of the tandem cell. One of the two materials could even be the low priced silicon, which would then be upgraded with an ultra-thin coating of III-V semiconductors. Teams at Fraunhofer ISE and the TU Ilmenau are working on designing cells, where III-V-semiconductors are combined with silicon, which will reduce costs significantly. Forecasts show that the generation of hydrogen with the help of solar energy and utilizing high-efficiency semiconductors may become economically competitive at efficiencies of around 15 percent corresponding to a hydrogen price of about four US dollars per kilogram.